Rotary Blood Pump

ABSTRACT

Various “contactless” bearing mechanisms including hydrodynamic and magnetic bearings are provided for a rotary pump as alternatives to mechanical contact bearings. In one embodiment, a pump apparatus includes a pump housing defining a pumping chamber. The housing has a spindle extending into the pumping chamber. A spindle magnet assembly includes first and second magnets disposed within the spindle. The first and second magnets are arranged proximate each other with their respective magnetic vectors opposing each other. The lack of mechanical contact bearings enables longer life pump operation and less damage to working fluids such as blood.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/504,233 of Wampler, et al. filed Sep. 18, 2003.

FIELD OF THE INVENTION

This invention relates to the field of rotary pumps. In particular, thisinvention is drawn to bearings for various rotor and impellerarchitectures.

BACKGROUND OF THE INVENTION

Typical rotary pumps utilize an impeller wherein the movement of theimpeller is constrained in five degrees of freedom (two angular, threetranslational) by mechanical contact bearings. Some working fluids maybe damaged by the mechanical contact bearings. Blood pumped throughpumps with contact bearings can experience hemolysis, i.e., damage toblood cells. In general, a hydraulically efficient and power efficientpump that can handle delicate working fluids such as blood is desirablefor some applications.

U.S. Pat. No. 6,234,772 B1 of Wampler, et al., (“Wampler”) describes acentrifugal blood pump having a repulsive radial magnetic bearing and anaxial hydrodynamic bearing. U.S. Pat. No. 6,250,880 B1 of Woodard, etal. (“Woodard”) describes a centrifugal blood pump with an impellersupported exclusively by hydrodynamic forces.

Both blood pumps are based on an axial flux gap motor design. The pumpimpeller carries the motor drive magnets thus serving as a motor rotor.In both cases, the drive magnets are disposed within the blades of theimpeller. Drive windings reside outside the pump chamber but within thepump housing that serves as the motor stator. Integration of the motorand pump enables the elimination of drive shafts and seals for thepumps. The pump/motors include a back iron to increase the magnetic fluxfor driving the impeller.

Both blood pumps suffer from hydraulic inefficiencies due at least inpart to the large, unconventional blade geometry required for disposingthe magnets within the impeller blades.

The natural attraction between the magnets carried by the impeller andthe back iron creates significant axial forces that must be overcome inorder for the pump to work efficiently. Hydrodynamic bearings can damageblood cells as a result of shear forces related to the load carried bythe hydrodynamic bearings despite the lack of contact between theimpeller and the pump housing. Thus exclusive reliance on hydrodynamicbearings may be harmful to the blood.

SUMMARY OF THE INVENTION

In view of limitations of known systems and methods, various“contactless” bearing mechanisms are provided for a rotary pump asalternatives to mechanical contact bearings. Various rotor and housingdesign features are provided to achieve magnetic or hydrodynamicbearings. These design features may be combined. The lack of mechanicalcontact bearings enables longer life pump operation and less damage toworking fluids such as blood.

In one embodiment, the pump includes a magnetic thrust bearing. The pumpincludes a pump housing defining a pumping chamber. The pump housing hasa spindle extending into the pumping chamber. A spindle magnet assemblycomprising first and second magnets is disposed within the spindle. Thefirst and second magnets of the spindle magnet assembly are arrangedproximate each other with their respective magnetic vectors opposingeach other. The pump includes a rotor having an impeller configured torotate about the spindle. A rotor magnet assembly comprising first andsecond magnets is disposed within a non-bladed portion of the rotor. Thefirst and second magnets of the rotor magnet assembly are arrangedproximate each other with their respective magnetic vectors opposingeach other. The relative orientations of the spindle and rotor magnetassemblies are selected so that the spindle and rotor magnet assembliesattract each other. The rotor may include a grooved bore. In variousembodiments, a hydrodynamic bearing is included for radial or axialsupport or both.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which likereferences indicate similar elements and in which:

FIG. 1 illustrates a cross-section of a pump having a passive magneticaxial bearing.

FIG. 2 illustrates one embodiment of the passive magnetic axial bearing.

FIG. 3 illustrates center and off-center placement of the passivemagnetic axial bearing.

FIG. 4 illustrates one embodiment of an impeller.

FIG. 5 illustrates one embodiment of the pump applied in a medicalapplication.

DETAILED DESCRIPTION

FIG. 1 illustrates one embodiment of a centrifugal blood pump. The pumpcomprises a housing 110 defining a pumping chamber 112 between an inlet114 and an outlet 116. Within the pumping chamber, a rotor 120 rotatesabout a spindle 130 protruding from a base of the pump housing. Therotor further comprises a bladed portion defining an impeller thatprovides the fluid moving surfaces. The impeller comprises one or moreblades 121 that move fluids when the impeller rotates.

The terms “rotor” and “impeller” may be used interchangeably in somecontexts. For example, when the rotor is rotating, the blade portion ofthe rotor is inherently rotating such that reference to rotation ofeither the impeller or the rotor is sufficient to describe both. Whennecessary, however, the term “non-bladed portion of the rotor” or “rotorexcluding the impeller” may be used to specifically identify portions ofthe rotor other than the blades. Each blade of the rotor may separatelybe referred to as an impeller, however the term “impeller” is generallyused to refer to a collective set of one or more blades.

The pump is based upon a moving magnet axial flux gap motorarchitecture. In one embodiment, the motor is a brushless DC motor.Drive magnets 122 carried by the rotor have magnetic vectors parallel tothe rotor axis of rotation 190. In the illustrated embodiment, the drivemagnets are disposed within a non-bladed portion of the rotor.

Drive windings 140 are located within the pump housing. Power is appliedto the drive windings to generate the appropriate time-varying currentsthat interact with the drive magnets in order to cause the impeller torotate. A back iron 150 enhances the magnetic flux produced by the motorrotor magnets. In one embodiment, either the face 124 of the bottom ofthe rotor or the opposing face 118 provided by the lower pump housinghave surfaces (e.g., 172) contoured to produce a hydrodynamic bearingwhen the clearance between the rotor and the housing falls below apre-determined threshold. In one embodiment, the pre-determinedthreshold is within a range of 0.0002 inches to 0.003 inches.

The natural attraction between the back iron 150 and the drive magnets122 carried by the rotor can create a significant axial load on therotor. This axial load is present in centrifugal pumps based on an axialflux gap motor architecture such as Wampler or Woodard. Woodard andWampler both rely on hydrodynamic thrust bearings to overcome this axialloading force. Despite the lack of contact, hydrodynamic bearings canstill damage blood cells as a result of shear forces related to the loadcarried by the hydrodynamic bearings.

The repulsive radial magnetic bearing of Wampler exacerbates the axialloads created by the magnetic attraction between the drive magnets andthe back iron. Although the repulsive radial magnetic bearing createsradial stability, it introduces considerable axial instability. Thisaxial instability can contribute further to the axial loading. Thisadditional axial loading creates greater shear forces for any axialhydrodynamic bearing that can cause undesirable hemolysis for bloodapplications. In addition, the power required to sustain thehydrodynamic bearing increases as the load increases. Thus highly loadedhydrodynamic bearings can impose a significant power penalty.

The blood pump of FIG. 1 includes a magnetic axial bearing that servesto reduce or offset the axial load imposed on the rotor by theinteraction between the drive magnets and the back iron. The axialmagnetic bearing is formed by the interaction between a spindle magnetassembly 160 disposed within the spindle and a rotor magnet assembly 180carried by the rotor. In the illustrated embodiment, the rotor magnetassembly 180 is disposed proximate the impeller, but the magnets of therotor magnet assembly are not located within the blades. A set screw 134permits longitudinal adjustment of the axial position of the axialmagnetic bearing by moving the spindle magnet assembly along alongitudinal axis of the spindle.

FIG. 2 illustrates one embodiment of the axial magnetic bearing. Therotor magnet assembly includes a first rotor bearing magnet 282 and asecond rotor bearing magnet 284 proximately disposed to each other. Thefirst and second rotor bearing magnets are permanent magnets. In oneembodiment, a pole piece 286 is disposed between them. A pole piece orflux concentrator serves to concentrate the magnetic flux produced byrotor bearing magnets 282 and 284. In an alternative embodiment, element286 is merely a spacer to aid in positioning the first and secondbearing magnets 282, 284 and does not serve to concentrate any magneticflux. In other embodiments, element 286 is omitted so that the rotormagnet assembly does not include a spacer or a pole piece element.

In one embodiment, elements 282 and 284 are monolithic, ring-shapedpermanent magnets. In alternative embodiments, the bearing magnets maybe non-monolithic compositions. For example, a bearing magnet may becomposed of a plurality of pie-shaped, arcuate segment-shaped, orother-shaped permanent magnet elements that collectively form aring-shaped permanent magnet structure.

The rotor axial bearing magnet assembly is distinct from the drivemagnets 222 carried by a portion of the rotor other than the blades 221.In the illustrated embodiment, the drive magnets are disposed within thenon-bladed portion 228 of the rotor.

The spindle magnet assembly includes a first spindle bearing magnet 262and a second spindle bearing magnet 264. The first and second spindlebearing magnets are permanent magnets. In one embodiment, a pole piece266 is disposed between them. Pole piece 266 concentrates the magneticflux produced by the spindle bearing magnets 262 and 264. In analternative embodiment, element 266 is merely a spacer for positioningthe first and second spindle bearing magnets and does not serve toconcentrate any magnetic flux. In other embodiments, element 266 isomitted so that the spindle magnet assembly does not include a spacer ora pole piece element.

In the illustrated embodiment, permanent magnets 262 and 264 arecylindrical. Other shapes may be utilized in alternative embodiments.The ring-shaped rotor magnets rotate with the impeller about alongitudinal axis of the spindle that is shared by the spindle bearingmagnet assembly.

The permanent magnets of each of the spindle and rotor bearingassemblies are arranged such that the magnetic vectors of the individualmagnets on either side of the intervening pole pieces oppose each other.Each side of a given pole piece is adjacent the same pole of differentmagnets. Thus the magnetic vectors of magnets 262 and 264 oppose eachother (e.g., N-to-N or S-to-S). Similarly, the magnetic vectors ofmagnets 282 and 284 oppose each other.

The orientation of the magnets is chosen to establish an axialattraction whenever the bearings are axially misaligned. Note that therelative orientations of the spindle and rotor magnet assemblies areselected so that the spindle and rotor magnet assemblies attract eachother (e.g., S-to-N, N-to-S). The magnet vector orientation selected forthe magnets of one assembly determines the magnetic vector orientationfor the magnets of the other assembly. Table 292 illustrates theacceptable magnetic vector combinations for the first and second rotorbearing magnets (MR1, MR2) and the first and second spindle bearingmagnets (MS1, MS2). Forces such as the magnetic attraction between theback iron and drive magnets that tend to axially displace the magnetbearing assemblies are offset at least in part by the magneticattraction between the axial bearings that provide an axial force torestore the axial position of the rotor.

FIG. 2 also illustrates wedges or tapered surfaces 272 that form aportion of a hydrodynamic bearing when the clearance between a face ofthe non-bladed portion of the rotor (see, e.g., bottom face 124 ofFIG. 1) and the back of the pump housing falls below a pre-determinedthreshold. In various embodiments, this pre-determined threshold iswithin a range of 0.0002 inches to 0.003 inches. Thus in one embodiment,the pump includes an axial hydrodynamic bearing. The surface geometryproviding the axial hydrodynamic bearing may be located on the rotor orthe housing.

Although the spindle magnet assembly is intended to provide an axialmagnetic bearing, the attractive force between the spindle and rotormagnet assemblies also has a radial component. This radial component maybe utilized to offset radial loading of the impeller due to the pressuregradient across the impeller. The radial component also serves as apre-load during initial rotation and a bias during normal operation toprevent eccentric rotation of the rotor about the spindle. Such aneccentric rotation can result in fluid whirl or whip which isdetrimental to the pumping action. The biasing radial component helps tomaintain or restore the radial position of the rotor and the pumpingaction, for example, when the pump is subjected to external forces as aresult of movement or impact.

Instead of a spindle magnet assembly interacting with a rotor bearingmagnet assembly to form the magnetic bearing, a ferromagnetic materialmight be used in lieu of one of a) the spindle magnet assembly, or b)the rotor bearing magnet assembly (but not both) in alternativeembodiments.

The magnetic bearing is still composed of a spindle portion and a rotorportion, however, one of the spindle and the rotor portions utilizesferromagnetic material while the other portion utilizes permanentmagnets. The ferromagnetic material interacts with the magnets to createa magnetic attraction between the rotor and spindle. Examples offerromagnetic materials includes iron, nickel, and cobalt.

In one embodiment, the ferromagnetic material is “soft iron”. Soft ironis characterized in part by a very low coercivity. Thus irrespective ofits remanence or retentivity, soft iron is readily magnetized (orre-magnetized) in the presence of an external magnetic field such asthose provided by the permanent magnets of the magnetic bearing system.

FIG. 3 illustrates various locations for the placement of the spindleportion of the magnetic bearing. In one embodiment, the spindle magnetassembly 360 is axially aligned with a longitudinal axis 390 of thespindle so that the spindle and spindle magnet assembly share the samecentral longitudinal axis. In an alternative embodiment, the spindlemagnet assembly is radially offset so that the spindle and spindlemagnet assembly do not share the same central axis. In particular, thelongitudinal axis 362 of the spindle magnet assembly 360 is displacedfrom the longitudinal axis 390 of the spindle. This latter positioningmay be desirable to provide some radial biasing force. A difference inpressure across the impeller tends to push the impeller radially towardsone side of the pump housing. This radial load may be offset at least inpart by offsetting the spindle magnet assembly.

Although the spindle and rotor magnet assemblies are illustrated ascomprising 2 magnetic elements each, the magnet assemblies may eachcomprise a single magnet instead. A greater spring rate may be achievedwith multiple magnetic elements per assembly configured as illustratedinstead of a single magnet per assembly. The use of two magneticelements per assembly results in a bearing that tends to correctbi-directional axial displacements from a position of stability (i.e.,displacements above and below the point of stability) with a greaterspring rate than single magnetic elements per assembly.

The magnetic force generated by the axial magnetic bearing will exhibita radial component in addition to their axial components. The radialcomponent will tend to de-stabilize the impeller. In particular, theradial component may introduce radial position instability for themagnetic bearing of either FIG. 1 or 2.

This radial instability may be overcome using radial hydrodynamicbearings. Referring to FIG. 1, the pump may be designed for a radialhydrodynamic bearing (i.e., hydrodynamic journal bearing) locatedbetween the spindle 130 and the rotor along the bore of the rotor. Theclearances illustrated in FIG. 1 are exaggerated. Hydrodynamic journalbearings require narrow clearances to be effective. In variousembodiments, the hydrodynamic journal bearing clearances range from0.0005-0.020 inches. The surface geometries suitable for axial (thrust)or radial (journal) hydrodynamic bearings may be located on either therotor or on an associated portion of the housing (or spindle). In oneembodiment, the surface geometry includes features such as one or morepads (i.e., a feature creating an abrupt change in clearance such as astep of uniform height). In alternative embodiments, the surfacegeometry includes features such as one or more tapers.

FIG. 4 illustrates one embodiment of the rotor 400 including animpeller. The impeller includes a plurality of blades 420 used forpumping the working fluid such as blood. The rotor includes a bore 410.The rotor bore is coaxially aligned with the longitudinal axis of thespindle within the pump housing. Drive magnets (not illustrated) aredisposed within the non-bladed portion 430 of the rotor (i.e., withinthe rotor but not within any blades of the impeller portion of therotor). The motor rotor and pump impeller are thus integrated so that adrive shaft is not required. Elimination of the drive shaft also permitselimination of shaft seals for the pump.

In one embodiment, the rotor has a grooved bore. In particular, the borehas one or more helical grooves 450. The bore grooves have a non-zeroaxial pitch. The groove is in fluid communication with the working fluidof the pump during operation of the pump.

FIG. 5 illustrates the pump 510 operationally coupled to move a workingfluid 540 from a source 520 to a destination 530. A first working fluidconduit 522 couples the source to the pump inlet 514. A second workingfluid conduit 532 couples the pump outlet 516 to the destination. Theworking fluid is the fluid moved by the pump from the source to thedestination. In a medical application, for example, the working fluidmight be blood. In one embodiment, the source and destination arearteries such that the pump moves blood from one artery to anotherartery.

Various “contactless” bearing mechanisms have been described asalternatives to mechanical contact bearings for rotary pumps. Inparticular, rotor, impeller, and housing design features are provided toachieve hydrodynamic or magnetic bearings. These design features may beused in conjunction with each other, if desired.

In the preceding detailed description, the invention is described withreference to specific exemplary embodiments thereof. Variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the invention as set forth in the claims.The specification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

1. A pump apparatus comprising: a pump housing defining a pumpingchamber, the pump housing having a spindle extending into the pumpingchamber; a spindle magnet assembly comprising a first and a secondmagnet disposed within the spindle, wherein the first and second magnetsare arranged proximate each other with their respective magnetic vectorsopposing each other; a rotor configured to rotate about the spindle,wherein the pump housing has a surface geometry structured to generate ahydrodynamic bearing for at least partially supporting the rotor duringrotation, the rotor including an impeller comprising at least one blade;and a plurality of drive magnets disposed within a non-bladed portion ofthe rotor.
 2. The apparatus of claim 1 further comprising: drivewindings carried by the pump housing, wherein the drive magnetsco-operate with the drive windings to rotate the rotor.